Several MPD subsystems can now read gzipped files; for example, the
database file can be gzipped.

When no track is found the error is now properly set.
Previously the calling function tried to log an undefined
error, which resulted in a crash. MPD falls back to
ffmpeg for unsupported tracks, such as ALAC.

This should fix issue 4051.

This plugin uses the MP4v2 library to play mp4/m4a files.
It is limited to file_decode.

The new class is pluggable, to prepare for gzipped database files.

For now, the TextFile class remains, and will be refactored away
later.

libfs.a depends on libsystem.a.

Support operating systems that don't have O_CLOEXEC.

Let the caller decide whether to strip.  Only remove \n and \r
(end-of-line markers).

Reduce the number of unnecessary memmove() calls.

Look at the buffer first, before trying to read from the file.  This
reduces overhead because we don't refill the buffer after every line.

Eliminate duplicate code.

The null byte is whitespace, too.

Eliminate the std::string overhead.

This was broken by commit 84d20d9e, which deleted the "!" from the
check.